Starting April 2017, Microsoft is rolling out a replacement for the Clutter folder in Outlook. This replacement is called the Focused Inbox. If you want to know a bit more, a couple Microsoft articles can be found here and here.
If you're like me and would rather not have this feature, here's how to turn it off:
Disable Focused Inbox for your whole organization
- Connect to Exchange Online via Powershell (more info)